Talk to me about vapor lock

RyanB

RyanB

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I've never felt like I've had a solid understanding of vapor lock.

The main questions are:
How does it occur (in both carburetorated and fuel injected engines)?
How to cure it?
How to prevent it from happening?

From what I understand, vapor lock would be less of a problem in a fuel injected system since the fuel is injected under pressure into each cylinder. I know this issue can also happen in carbureted engines as well, but my understanding about how it occurs is gray.

Any insight is appreciated
Thanks.
 
Vapor lock results from fuel boiling. The operational result is little or no liquid fuel available for injection or filling the carburetor bowl.

Hot fuel and high altitude can contribute to vapor forming (boiling) in the fuel line while the engine is running. One symptom is fluctuating fuel pressure. Some aircraft POHs say to run the boost pump (increase fuel system pressure) if fuel pressure is not steady.

Side note, aircraft (and most auto) fuel injection systems don't inject fuel into each cylinder. They deliver fuel to the intake manifold usually very near the cylinder intake valve.
 
My limited understanding is fuel is a liquid and fuel systems are designed for liquids. vapor lock occurs when the fuel changes states to a gas(vaporizes) and can't be delivered. In other words, your fuel pump can't pump vapor. I am sure I am wrong though and someone will be along shortly to explain how wrong I am.
 
It should only occur in a fuel injected engine. Fuel lines to the cylinders are routed over the top of the engine and get hot after shutdown creating pockets of vapor in the lines. Start-up fuel availability is irregular as a result. I've never heard of vapor lock in an updraft carb induction.

Fuel return lines and purge valves are sometimes used to manage fuel line heat and pressure. Pressurized fuel lines can cause fuel leaks through the injectors and into the cylinders at shutdown. You may see "zero leak" fuel servos on exp engines. Shutdown fuel pressure is what that's meant to manage.
 
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A hydraulic pumping engineer described it to me and I will paraphrase a bit. Decrease the pressure enough on a liquid and bubbles form. The same thing happens when you hold your arm under water in the swimming pool and move it extremely fast, bubbles form in the area immediately behind your arm in the turbulent area. Gasoline is sensitive to heat and pressure changes that along with the convoluted maze of tubing and fittings that the fuel has to travel through to get from the tanks to the engine is not ideal. While the atmosphere keeps the fuel in the tanks bubble-free, heating and the numerous bends in the lines and fittings create turbulence in the flow, the higher you climb the more likely the turbulence will create "vapor". There are precautions used to help combat vapor such as header tanks, a small tank near the engine (as close as practical) where the fuel flow is near free of turbulence and any vapor can be vented back to the main tank. Also, cooling air blowing on fuel pumps or insulation on fuel lines can reduce the tendency to vaporize.
 
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Thanks all.

If such a situation occurs inflight, what is the best course of action to cure it (FI and carbureted)? I understand it to be that it's unlikely to occur in flight, but more apt to happen during startup on the ground, is this correct?
 
The vapor lock ya gotta watch out for is called running out of gas. In flight it can be bad.
 
Ryanb said:
Thanks all.

If such a situation occurs inflight, what is the best course of action to cure it (FI and carbureted)? I understand it to be that it's unlikely to occur in flight, but more apt to happen during startup on the ground, is this correct?
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By far the most common occurrence (as described by stewartb above) is a hot start on fuel injected engines. The engine is off, so the pumps aren't pressurizing anything. The engine is hot and the fuel lines run over the engine (with no cooling airflow, too) and the heat coupled with the reduction in pressure causes the fuel to change to a gas. So you have bubbles in the lines when you try to start. There are a variety of ways to deal with it.

I've never heard of vapor lock on a running air craft engine in flight, but I've only been doing this for a dozen years and only have ~250 ish hours, mostly in carberated air craft.
 
Here's an easy read article you may like. https://generalaviationnews.com/2011/06/26/vapor-lock-ethanol-and-ultralights-2/

My new plane uses fuel injection with a zero leak down servo and no return lines, no purge valve. Both my electric high pressure fuel pump and the mechanical fuel pump are shrouded and cooled by blast tubes. FI systems and heat don't play well together. In flight the pressure cowl air flow prevents hot fuel lines but a hot engine on the ground will cook fuel lines and heat up my fuel pumps. My vapor potential is ground based. I have no concern about it in flight.

Here's a pic of my engine the day I uncrated it. Fuel distribution spider and the individual fuel lines are all exposed to shutdown overheating. In flight all the cowl flow is top of engine down. Remove that air flow and those cylinders are radiating heat upwards.
 

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Stewartb said:
Here's an easy read article you may like. https://generalaviationnews.com/2011/06/26/vapor-lock-ethanol-and-ultralights-2/

My new plane uses fuel injection with a zero leak down servo and no return lines, no purge valve. Both my electric high pressure fuel pump and the mechanical fuel pump are shrouded and cooled by blast tubes. FI systems and heat don't play well together. In flight the pressure cowl air flow prevents hot fuel lines but a hot engine on the ground will cook fuel lines and heat up my fuel pumps. My vapor potential is ground based. I have no concern about it in flight.

Here's a pic of my engine the day I uncrated it. Fuel distribution spider and the individual fuel lines are all exposed to shutdown overheating. In flight all the cowl flow is top of engine down. Remove that air flow and those cylinders are radiating heat upwards.
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Thanks Stewart. Enjoyed the article as well.
 
Liquids have boiling points. You can get a liquid to boil if you raise its temperature to its boiling point, or if you reduce the atmospheric pressure on it without raising its temperature. Remember the old water-in-a-bell-jar demonstration in school? Water at room temperature, with enough air sucked out of the bell jar, will boil. If the water is warmer, it takes less vacuum.

The fuel in the airplane has to deal with heat and sometimes with pressure drops. Hot days and heat under the cowl warms the fuel. A fuel pump sucking fuel uphill will lower the pressure on the fuel, as in a low-wing airplane with the boost pump off and the engine's pump pulling the fuel up. Pull on the fuel on a hot day and you could get vapor lock as the fuel upstream of (below) the pump starts to boil and the pump gets nothing but vapor. Use Mogas and it gets worse, since Mogas has higher volatility, another way of saying that its boiling point is lower. Old cars had this problem when they had only engine-driven pumps. Lifting the fuel a few inches would do it. When they stuck electric pumps in the tank it went away.

Injected engines have their injection lines above the engine. Shut it down on a hot day in a quiet place and listen at the cowl opening in front. You'll hear the fuel boiling in those lines, and it's being forced out into the intakes so that it tends to flood the engine. Hot start.
 
It can happen in flight. Early Comanches had a potential problem with vapor lock in flight when the boost pumps were left on to long. Fuel pressure from the boost pumps would overcome fuel pressure from the engine pump and allow hot fuel to form a bubble at the "t" where the systems join. When the boost pumps were shut off, vapor lock. That's why it's recommended that the pumps be shut off in the climb at the earliest opportunity safety allows. Probably also why Comanche's are not eligible for an autofuel STC available with the Lycoming O-540.
 
Ryanb said:
Thanks all.

If such a situation occurs inflight, what is the best course of action to cure it (FI and carbureted)? I understand it to be that it's unlikely to occur in flight, but more apt to happen during startup on the ground, is this correct?
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In a Cessna, that is what the red half of the fuel pump switch is for. (high boost pump)

I had a burble in a 210 taking off from Mobil Downtown. About 500 feet over the bay the engine started surging. My first thought was, "I am going to miss lunch", my second thought was "I am going to get wet" and my next thought was turn on the red switch. I did and the problem vanished.
 
Just a point of reference:

The early bonanza was designed to run what was available- car gas. Two primary elements appear to be what keeps the system from vapor locking running car gas: the wobble pump and the fuel return line. (Ok, outside the low compression engine.)

The wobble pump is the only way to start a hot engine on a hot ramp this time of year. When I had a -35 with the electric fuel aux pump, it seemed like the fuel (near?) the pump would vaporize rendering it a noise maker. The wobble with the large plunger and insides will push fuel any time.

The fuel return line is the biggie when in the air. It appears the extra (3 gph?) fuel transfers heat and cools the plumbing under the cowling. I know a guy that added an STC to the old E225 which required the removal of the fuel return line and the result was vapor lock at altitude when running car gas. That will get your attention!

How to prevent vapor lock? Got me- other than learning as much as you can about your bird. In my case- that archaic wobble is the solution.

Just hoping I don't ever have to run the wobble and do a manual gear extension. . . .
 
Had vapor lock in an Aerostar and in a Cirrus and Bonanza. I do learn eventually....
In all cases hot fuel, hot day, climb to 10K plus altitude and you will see it. Solution is turn on the boost pump.

Tim

Sent from my LG-H631 using Tapatalk
 
tspear said:
Had vapor lock in an Aerostar and in a Cirrus and Bonanza. I do learn eventually....
In all cases hot fuel, hot day, climb to 10K plus altitude and you will see it. Solution is turn on the boost pump.
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Happened to me for the first time flying to Telluride recently.. set the timer in cruise and waited a full 30 minutes but when I shut the pump off I got rude and sudden loss of power. Pump went right back on!!
 
The cure for this is a system that constantly circulates the fuel (like the one in your car) where excess fuel is returned to the tank, rather than "dead headed" at its destination. They've added such a system to some Rotax engines to prevent vapor lock.
 
Bypasses, not circulates. That’s how Continental’s FI works. It requires an accumulator tank and fuel return lines into the cabin.
 
Stewartb said:
Bypasses, not circulates. That’s how Continental’s FI works. It requires an accumulator tank and fuel return lines into the cabin.
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Thus, "circulates": Tank>fuel pump>tank, unless burned in the engine. Cools things down nicely. BTW, DeLoreans were famous for vapor locking.
 
Kenny Phillips said:
The cure for this is a system that constantly circulates the fuel (like the one in your car) where excess fuel is returned to the tank, rather than "dead headed" at its destination. They've added such a system to some Rotax engines to prevent vapor lock.
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You have to pick your poison. You either need to circulate and have a return run a fuel which makes the plumbing and everything more complicated and more failure points or you need to pump at the fuel tank which as another failure point the only solution is a gravity-fed which is strong enough to maintain pressure.

Sent from my SM-J737T using Tapatalk
 
I recently had a choice of FI systems for my exp. Return line with bypass? Return line with a purge valve? No return line and deal with it? I chose the latter. On purpose. Hot starting by priming and using flooded start procedures works every time. Vapor “lock” is a misnomer. Nothing is locked. It just requires vapor management.
 
Carb engine get it too. The Cherokee 180 auto gas STC requires the addition of a second electric fuel pump to prevent vapor lock.
 
Kenny Phillips said:
The cure for this is a system that constantly circulates the fuel (like the one in your car) where excess fuel is returned to the tank, rather than "dead headed" at its destination. They've added such a system to some Rotax engines to prevent vapor lock.
Click to expand...

Carbureted engines absolutely will get it too. Fuel boiling anywhere in a fuel line (e.g. over the top of the engine before the carburetor) will disrupt fuel flow.

In terms of Rotax, they’ve issued a service bulletin to mandate a return fuel line, but unfortunately there have already been cases where vapor lock still occurred.

I think the key here is to keep the fuel lines as cool as possible. If after a leg of a flight where you land and have a full stop, it may not be a bad idea to open the cowl and allow the heat from the engine escape.
 
What carb engine has a fuel line laying on top of the engine?
 
Stewartb said:
What carb engine has a fuel line laying on top of the engine?
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Mine.
Fuel line runs from the gascolator at the upper port side of the firewall to the mechanical fuel pump on the forward starboard side of the engine, then back to the carburetors.
 
Kenny Phillips said:
The cure for this is a system that constantly circulates the fuel (like the one in your car) where excess fuel is returned to the tank, rather than "dead headed" at its destination. They've added such a system to some Rotax engines to prevent vapor lock.
Click to expand...

For what’s worth, many car fuel systems are returnless these days. Modern EFI has been able to compensate for vapor lock fairly well but not always. Under some conditions my returnless Audi will hot start a bit weird.

My solution? Use the older fuel system components that included a fuel return line. ;)
 
Stewartb said:
What carb engine has a fuel line laying on top of the engine?
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Rotax, it's an odd setup for an airplane as the fuel line runs all the way across the top of the engine to the pump that is mounted up front. There is also a large in line fuel filter sitting up there that is a perfect heat sink on a parked aircraft. I'm not sure whey they do that since the carburetors are at the back. As has been noted vapor lock occurs on the suction side of a fuel pump, it doesn't happen on the downstream discharge side.
 
Some people get this, vapor lock, when reading spin zone type posts.
 
mondtster said:
For what’s worth, many car fuel systems are returnless these days. Modern EFI has been able to compensate for vapor lock fairly well but not always. Under some conditions my returnless Audi will hot start a bit weird.

My solution? Use the older fuel system components that included a fuel return line. ;)
Click to expand...

Modern carts have in tank pumps, it's not possible for them to get vapor lock. Hard starting is not vapor lock, with vapor lock the system is truly "locked" the fuel pump cannot get fuel to deliver and will not until the system cools or you re-prime it somehow.
 
Silvaire said:
Modern carts have in tank pumps, it's not possible for them to get vapor lock.
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"Hot Fuel Compensation" is one of the modules in the automotive powertrain control software that I used to work on not too many years ago.

Returnless fuel systems are more popular in "modern" cars (and, I assume, carts :) ) because you get less heat in the fuel tank which leads to less fuel vapor in the carbon canister which makes it easier to pass the S.H.E.D. test.
 
Stewartb said:
What carb engine has a fuel line laying on top of the engine?
Click to expand...

Folks beat me to the answer. Almost all aircraft using Rotax engines have fuel lines across the top of the engine due to the mechanical fuel pump location near the gearbox for the propellor.

Add the further complexity of ethanol MOGAS (which more easily vaporizes) and tight cowling on many light sport aircraft, it makes one take pause when the weather is warm. Use 100LL or MOGAS, and how long do I have to wait to take off? If it’s too long, when do I chose to abort the flight and wait until it’s less busy (and/or less hot?)
 
Stewartb said:
I recently had a choice of FI systems for my exp. Return line with bypass? Return line with a purge valve? No return line and deal with it? I chose the latter. On purpose. Hot starting by priming and using flooded start procedures works every time. Vapor “lock” is a misnomer. Nothing is locked. It just requires vapor management.
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The earlier 172R and S models had no return systems. IO-360 engines. Some of them experienced vapor lock and Cessna issued an SB outlining the means of adding a return line and check valve. Not an AD (at least not yet) so it's not mandatory. The old carbureted 172s used only gravity feed for fuel flow, so vapor lock wasn't an issue. It isn't likely due to the small head pressure all the way from the tanks, and the abundant airflow though the cowling keeps the FWF stuff cool enough. There's no engine-driven fuel pump (like the injection system has) to pull on the fuel and reduce its pressure and making vapor lock more likely. The injected system needs a boost pump under the cabin floor to maintain pressure on the FWF lines in the event of engine hiccupping, and for priming for starting.

Injected engines don't get vapor lock between the engine-driven pump and the fuel servo and injectors. Way too much pressure on it. It's the fuel strainer and line to the pump that's at risk.
 
Not aviation related, but I have a ski boat with a 350 mag mpi that is TERRIBLE to vapor lock. There are a few things I’ve found that help alleviate it but I don’t know if they apply to you or airplanes. Letting the engine idle for a few minutes before shutting it down can help, and help even more if done with the engine cover raised up. There is a valve on the fuel rail that can be pushed to let out any vapor that has built up in them. There are also some aftermarket systems that cycle the unburned fuel back into the gas tank as was previously mentioned in this thread. However, the easiest thing I’ve found to do is use non ethanol fuel only. Did that all last summer and didn’t have a single problem with vapor lock. I have also never had or heard of an engine vapor locking while running, but I don’t doubt that it’s possible.

Edit: just realized how old this thread is. Hope you’ve got it figured out by now!
 
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